Radiography and fluoroscopy are well known diagnostic imaging techniques.
In a conventional radiography system, an x-ray source is actuated to direct a divergent area beam of x-rays through a patient. A cassette containing an x-ray sensitive phosphor screen and light and x-ray sensitive film is positioned in the x-ray path on the side of the patient opposite the source. Radiation passing through the patient's body is attenuated in varying degrees in accordance with the various types of tissue through which the x-rays pass. The attenuated x-rays from the patient emerge in a pattern, and strike the phosphor screen, which in turn exposes the film. The x-ray film is processed to yield a visible image which can be interpreted by a radiologist as defining internal body structure and/or condition of the patient.
In conventional fluoroscopy, a continuous or rapidly pulsed area beam of x-rays is directed through the patient's body. An image intensifier tube is positioned in the path of the beam opposite the source with respect to the patient. The image intensifier tube receives the emergent radiation pattern from the patient, and converts it to a small, brightened visible image at an output face. Either a mirror or closed circuit television system views the output face and produces a dynamic real time visual image, such as on a CRT, for interpretation by a radiologist.
More recently, digital radiography and fluoroscopy techniques have been developed. In digital radiography, the source directs x-radiation through a patient's body to a detector in the beam path beyond the patient. The detector, by use of appropriate sensor means, responds to incident radiation to produce analog signals representing the sensed radiation image, which signals are converted to digital information and fed to a digital data processing unit. The data processing unit records, and/or processes and enhances the digital data. A display unit responds to the appropriate digital data representing the image to convert the digital information back into analog form and produce a visual display of the patient's internal body structure derived from the acquired image pattern of radiation emergent from the patient's body. The display system can be coupled directly to the digital data processing unit for substantially real time imaging, or can be fed stored digital data from digital storage means such as tapes or discs representing patient images from earlier studies.
Digital radiography includes radiographic techniques in which a thin fan beam of x-rays is used. In this technique, often called "scan (or slit) projection radiography" (SPR) a fan beam of x-rays is directed through a patient's body. The fan is scanned across the patient, or the patient is movably interposed between the fan beam x-ray source and an array of individual cellular detector segments which are aligned along an arcuate or linear path. Relative movement is effected between the source-detector arrangement and the patient's body, keeping the detector aligned with the beam, such that a large area of the patient's body is scanned by the fan beam of x-rays. Each of the detector segments produces analog signals indicating characteristics of the received x-rays.
These analog signals are digitized and fed to a data processing unit which operates on the data in a predetermined fashion to actuate display apparatus to produce a display image representing the internal structure and/or condition of the patient's body.
One of the advantages of digital radiography and fluoroscopy is that the digital image information generated from the emergent radiation pattern incident on the detector can be processed, more easily than analog data, in various ways to enhance certain aspects of the image, to make the image more readily intelligible and to display a wider range of anatomical attenuation differences.
An important technique for enhancing a digitally represented image is called "energy subtraction".
Energy subtraction exploits energy-related differences in attenuation properties of various types of tissue, such as soft tissue and bone, to derive a material-specific image, mapping substantially only a single material in the body.
It is known that different tissues, such as soft tissue (which is mostly water) and bone, exhibit different characteristics in their capabilities to attenuate x-radiation of differing energy levels.
It is also known that the capability of soft tissue to attentuate x-radiation is less dependent on the x-ray's energy level than is the capability of bone to attenuate x-rays. Soft tissue shows less change in attenuation capability with respect to energy than does bone.
This phenomenon enables performance of energy subtraction. In practicing that technique, pulses of x-rays having alternating higher and lower energy levels are directed through the patient's body. When a lower energy pulse is so generated, the detector and associated digital processing unit cooperate to acquire and store a set of digital data representing the image produced in response to the lower energy pulse. A very short time later, when the higher energy pulse is produced, the detector and digital processing unit again similarly cooperate to acquire and store a set of digital information representing the image produced by the higher energy pulse.
In early energy subtraction techniques, the values obtained representing the lower energy image were then simply subtracted from the values representing the higher energy image.
Since the attenuation of the lower energy x-rays by the soft tissue is about the same as the attenuation of the higher energy x-rays, subtraction of the lower energy image data from the higher energy image data approximately cancels out the information describing the configuration of the soft tissue. When this information has been so cancelled, substantially all that remains in the image is the representation of bone. In this manner, the contrast and visibility of the bone is substantially enhanced by energy subtraction.
Energy subtraction has the advantage of being substantially not subject to motion artifacts resulting from the patient's movement between exposures. The time separating the lower and higher energy image acquisitions is quite short, often less than one sixtieth of a second.
Details of energy subtraction techniques in digital radiography and fluoroscopy are set forth in the following technical publications, all which are hereby incorporated specifically by reference:
Hall, A. L. et al: "Experimental System for Dual Energy Scanned Projection Radiology". Digital Radiography proc. of the SPIE 314: 155-159, 1981; PA0 Summer, F. G. et al: "Abdominal Dual Energy Imaging". Digital Radiography proc. SPIE 314: 172-174, 1981; PA0 Blank, N. et al: "Dual Energy Radiography: a Preliminary Study". Digital Radiography proc. SPIE 314: 181-182, 1981; and PA0 Lehman, L. A. et al: "Generalized Image Combinations in Dual kVp Digital Radiography", Medical Physics 8: 659-667, 1981.
The above incorporated article by Lehman, et al describes more recently conceived techniques for modifying the above described simple subtraction technique to enhance the quality of the energy subtracted image.
Dual energy subtraction has been accomplished, as noted above, by pulsing an x-ray source in a digital scanning slit device at two kVp's, typically 120 and 80 kVp, and synchronizing the pulses with a rotating filter which hardens the high kVp pulses by filtering out the lower energy x-rays. This results in the patient and x-ray detector sequentially seeing high energy and low energy beams from which the mass per unit area of bone and soft tissue can be solved for.
More recently, it has been proposed in energy subtraction to utilize a particular type of dual energy detector assembly which can produce separate signals representing each of lower and higher x-ray energy incident on the detector. Such a detector assembly enables the practice of energy subtraction without the necessity for switching kVp x-ray output levels, or employing other means for periodically attenuating the x-ray beam, such as rapid interposition and removal of a filter to and from the x-ray path. Such a detector employs a dual layer of phosphor-detector elements, wherein the phosphor material of a first, or front, layer preferentially responds to energy of a relatively lower energy value. A second, or rear, detector layer preferentially responds to x-ray energy in a higher range. Such a detector, and its method of use, is described in published European Patent Application No. 83307157.4 published on Aug. 8, 1984 by Gary T. Barnes, which published application is hereby expressly incorporated by reference.
It is often desirable, in medical diagnostic imaging, to detect characteristics of lesions such as nodules within the human body. Such nodules often are found in the chest area of the patient, including in the lungs. The nodules are typically composed of both soft tissue and calcium, the calcium occurring in differing amounts among different nodules.
The ratio of soft tissue to calcium in a lesion such as a nodule can carry highly significant medical implications. For example, a high degree of calcification of a nodule bears a strong correlation with the likelihood that the nodule is not malignant. Calcification, however, can be an indication of disease other than cancer. Moreover, the progression of calcification often carries significant medical implications as well. Additionally, the relative location of calcification within a nodule can have important significance.
One of the problems in attempting to quantify calcification of a lesion or other structure or object of interest within the body is that normal anatomy includes much calcified structure, such as bone. Additionally, even soft tissue in the body contains trace amounts of calcium which shows up in energy subtraction detection as a measurable background value. Accordingly, a calcified lesion or other structure, whose calcium content is sought to be detected, is almost always overlying or overlain by other interfering calcium, making it difficult to distinguish between the calcium of the lesion or structure sought to be investigated and the other calcium of the body.
A proposal has been made to utilize computerized tomography (CT) equipment for detecting calcification.
See Siegelman, S. S., et al., "CT Of The Solitary Pulmonary Nodule", American Journal of Roentgenology, 1980; 135: 1-13; Tarver, R. D. et al., "Experimental Lung Nodule Model: Numbers, Nodule Size, and Actual Calcium Content", Journal of Computer Assisted Tomography, 1983; 7 (3): 402-406.
It is not believed, however, that such methods actually directly determined the quantification of calcification of nodules.
Another proposal involved measuring the difference in optical density on conventional radiographic film exposures between a location within a nodule and a location just outside the nodule, which was said to "allow a quantitative estimate of . . . [calcification] within the nodule." This method, however, yielded only an estimate, and did not directly measure calcification. Also, it was stated to be inapplicable when another calcified structure, such as a rib, overlay the nodule. See Kruger, R. A. et al "Dual Energy Film Subtraction Technique for Detecting Calcification In Solitary Pulmonary Nodules", Radiology, 1981; 140: 213-219.
It is believed that the only known method of directly determining calcification of a nodule, in a practical sense, has been the surgical excision and subsequent chemical assay of the nodule.
It is an object of this invention to provide a non-invasive method and system for accurately determining the quantity of calcium or other substance present in a lesion or structure of an animal body, and to distinguish the substance in the lesion or structure of interest from other presence within the body of the same substance.